In the manufacture of many silicon devices, in particular power devices, one starts with a low resistivity substrate wafer onto which an epitaxial layer is grown by chemical vapor deposition (CVD). Typically, for power devices, an epitaxial layer is approximately 50-100 .mu.m thick and it provides the basis for the active part of the device. Accordingly, the epitaxial layer is doped during growth from the gas phase in a very precise and well-defined way in order to obtain the required doping profile across the epitaxial layer. For example, for an NPN transistor, located near the substrate, one has an intrinsic layer as a collector field region followed by a P-type base region and an N-type emitter region. It is very important to control and monitor the doping profile for such a transistor very accurately. Monitoring the doping profile can be done, for example, by sacrificing one wafer from each batch that is being processed and grinding a beveled edge through the epitaxial layer of such a wafer. On the bevel surface of the sacrificed wafer, the conductivity is measured across the epitaxial layer with a 2-point probe technique by which 10-100 points are used for the conductivity measurements. Even though the apparatus used to make the conductivity measurements is mechanized and quite automatic, the beveling and measuring procedure is both time consuming as well as being costly. This is so because at every point of measurement across the layer one has to position the contacts, take a current-voltage reading, raise the contacts, move them to the next point for measurement, and so on. See Modern Microelectronics by Max Fogiel, Research and Education Association, New York, N.Y., pp. 412-413 for a description of a typical resistivity measurement using 4-point and 2-point probe techniques.
There is a need in the art, nevertheless, for a procedure for determining doping profiles without requiring point contacts that have to be moved across the layer.